Yeah that's fine

The place to submit time trials is mariokart64.com
Unfortunately you're looking at the wrong time though. You're playing on an NTSC system while comparing to the PAL world record.
The current NTSC record is 26.01. The current PAL WR of 30.92 is equivalent to a 25.72 on NTSC.

I'd recommend turning a little to the opposite direction before hopping into the slide. Like if you were on the start of Luigi Raceway and wanting to do left-facing MTs, before jumping into the slide, angle your kart a bit to the right so that your angle when you complete the MT isn't quite as bad. Over time you'll improve and won't have to angle the kart as much before jumping into the slide.

Do you just mean MTs in general or specifically SSMTs (straight stretch MTs)?

The time trial world records are significantly stronger than you might think. You're playing on the NTSC version of the game but you were looking at the PAL WR time which is equivalent to a 1'37"95 on NTSC.

The best run actually done on the NTSC system is a 1'38"17

No, you cannot use savestates.

Nope, you can't pause the timer.

Yeah, nothing weird for the Switch release as far as I could tell (at least in single player)

Yeah you'd just submit it under VC

Yes, you can submit both and they'll get filtered into those sections if someone is viewing those regions directly

Emulator times are not allowed on the time trial rankings

Yeah happens all the time. Tunnel floor is all messed up at the entrance

Yes this is very well known and is even mentioned in the time trial rules: https://64.mk/mk64/rules.html
That being said, it's considered a shortcut which makes it useless since there is a much better shortcut in the cave.

Looks like it was for Double Dash

https://beckabney.com/mk64/onetries/

You have to finish the race with both players anyways so it's not very fast. All of this has already been thought of. There are 2p GP ILs over at https://www.mariokart64.com/mk64/150cc/
There have been some co-op runs done before though just for the fun of it.

All of our 150cc categories are specifically for 1p GP mode so not really much you can do there

Project 64 1.6 would be the one to use. Also probably set Counter Factor to 1 in Options > Settings > Rom Settings.

You can just submit your run again I think. Otherwise I can always just edit it I suppose.

Also PAL and NTSC aren't split for full game runs since they are done with RTA timing. So playing NTSC would obviously be recommended. (Our leaderboards are already segmented a ton as is with all the different consoles + skips/no skips). You can always adjust the filters to show PAL only if you'd like.

No, Wii U VC is not allowed for time trials.
Time trial website: https://mariokart64.com
Time trial rules: https://64.mk/mk64/rules.html

I can't imagine someone else hasn't suggested this before but it would be great to have an way to only allow centiseconds instead of milliseconds for submissions.
95% of the submissions I get to Mario Kart 64 with milliseconds on them end up with a 0 in the front of them that has to be edited because the person submitting only submitted 2 digits.
For example 40'00"55 turns into 40'00"055 since the input box is milliseconds